I guess there are various reasons such battery utilities are set up in the vicinity of where they are supposed to provide back-up and storage.

Reasons may include:

a) They can easily use already existing local grid systems (power stations and transformers);

b) They can locally stabilize local grid (sub-) systems in case of fluctuations in power, thus alleviating cascading burdens on the whole grid system;

c) They may lower anciliary services and their costs (less power is needed to overcome larger distances of distribution).

OTOH: If there is an emergency and a fire breaks out and the utility is let burn down, toxic chemicals will spread via wind regardless. So putting them 20 miles away may simply not be enough to protect people's health to outweigh the negative impacts on supply and grid stability. (Remember how the Flint water poisoning couldn't be contained to a local area.)